Because the voltage to run the EL wire is different than the batteries that power it, so you have to convert the voltage in order to match that of the EL wire. Otherwise you could burn out the wire, or not have a high enough voltage to properly light it up. It’s the same thing when you want to run a laptop’s wall charger (which is 120v) off your car’s cigarette lighter power plug (which runs at 12v, but higher amperage). You have to get an inverter to convert it